This invention is concerned with a method for the preparation of additives for influencing the electrocrystalline growth in aprotic oxygen- and water-free organo-aluminum electrolyte media, which is characterized by the feature that a compound, which contains a sulfonimide or sulfonamide group as the reactable group, is reacted with at least one organo-aluminum compound of the general formula AlR.sub.3, where R is an alkyl radical, particularly ethyl, in the molar ratio of from about 1:1 to 1:6. The reaction takes place preferably in the presence of an inert organic solvent such as, for instance, toluene or xylene.
Suitable sulfonimides are, for example: ##STR1##
o-benzoic acid sulfonimide and N-benzoylbenzenesulfonimide have been found to be particularly suitable reaction components.
In a concentration range of 10 to 100 g/l of this additive of o-benzoic acid sulfonimide and aluminum triethyl, a brightness (reflectivity) of 80% was obtained, i.e., the reflectivity corresponded to about 80% of that of a silver mirror.
The reaction is performed by apportioned addition of the aluminum trialkyl, diluted with an organic solvent, to the organic, sulfonimide group-containing compound. Temperatures of up to about 100.degree. C, and preferably, of about 70.degree. to 80.degree. C, should be used in order to avoid local overheating through an exothermic reaction of the reaction partners. Because of the chemical properties of the aluminum alkyl compounds, the reactions must be performed in a dry inert gas, i.e., with the exclusion of air and moisture.
Suitable organo-aluminum compounds are aluminum trialkyls, preferably with alkyl radicals from C.sub.1 to C.sub.4 . Particularly well suited is aluminum triethyl. The additives, i.e., the brighteners, may also be isolated from the solution as defined compounds.
The additives obtained in accordance with the invention act as brighteners or inhibitors of crystaline growth with aprotic, oxygen-and water-free organo-aluminum electrolyte media. If added in solid form, or preferably, in solution, they make possible the electrodeposition of aluminum on metals and conductive materials in shiny or mirror-like form. Particularly well suited is the additive obtained from 1 mole o-benzoic acid sulfonimide and 2 moles aluminum triethyl. The additives as well as the organo-aluminum electrolyte can be dissolved in inert, water-free aprotic solvents, e.g., xylene or toluene. Through the use of the additives prepared by the method according to the invention, in organo-aluminum electrolytes, one obtains a highly-adhering, shiny and mirror-like aluminum deposit directly on metals and conductive materials.
The bright aluminum layers produced, using the additives prepared in accordance with the invention, find use as mirrors and reflectors for light and heat radiators as well as for decorative purposes and for corrosion protection, and in general for surface finishing. If required, the layers can also be coated with a hard, electrically insulating layer by means of anodic oxidation, and/or stained.